(1) Field of the Invention
The invention relates to methods and apparatuses for controlling the flow of high energy electrical pulses to a load using a switch having a plasma cathode which has rapid start up capability and requires no heater power as does, for example, a conventional thyratron. The invention finds its application in those systems wherein short-time-duration high energy pulses are delivered to a load, for example, ion engines, radar systems, lasers, particle accelerators, and those electromagnetic pulse testing systems which simulate the electromagnetic fields developed in a nuclear explosion.
(2) Description of the Prior Art
Various high energy short-time-duration pulse controlling systems are known in the art. Gas tubes and vacuum tubes are usually depended upon for their operation.
High vacuum tubes (hard tubes) rely on the well known thermoionic emission from a cathode which produces a limited number of charge carriers (no ions). The obtainable current density is limited by an electron space charge at the cathode; however, the advantage is that no ions exist to form a positive space charge around the control grid when it is pulsed negative. Hence, hard tubes may be turned on and off, even in the condition of current flow.
Gas-filled tubes generate their charge carriers from ionizing electron-molecule collisions which produce an electrically neutral plasma with electrons and positive ions moving in opposite directions. Conduction may be held off by a control grid in the absence of current flow, but once switched on, the ions form a space charge around any grid which tries to go negative so that its controlling electric field is cancelled. This results in an out of control arc. The gas filled tubes provide on-switching capability of almost unlimited currents, but must be externally turned off until recombination dissipates the plasma. Only then may the control grid holdoff function be re-established. Statistical ionization processes can also be important when jitter requirements are severe, or when trigger-to-breakdown times must be 0.5 .mu.second or less. Recent use of hydrogen and grounded-grid designs have shortened conventional thyratron ionization and deionization times, but hydrogen clean up presents a problem, and heated reservoirs are needed to attempt to maintain a suitable equilibrium operating pressure over the tube's lifetime.
The switch described herein offers and major advantages of both hard tubes and thyratrons. It yields kilohertz repetition capability, high power handling, high voltage hold off, nanosecond rise time, and has an impedance distribution to match any transmission line geometry. It suffers none of the thermionic limitations characteristic of hard tubes.
Reference is made to the following works for those relationships, equations, and definitions which are used hereinafter:
Gaseous Conductors--Theory and Engineering Applications, James D. Cobine PhD., Dover Publications Inc. 1958 edition.
Basic Data of Plasma Physics, Sanborn C. Brown, M.I.T. Press 1959 edition.
Continuous Uniform Excitation of Medium Pressure CO.sub.2 Laser Plasmas by Means of Controlled Avalanche Ionization, Alan E. Hill, Applied Physics Letters, vol. 22, no. 12, June 1973.